Why the Cardinals did it: The Cardinals have been looking to add a superstar-caliber player for a few seasons now and finally got one in Goldschmidt, a six-time All-Star who finished sixth in the MVP voting this past season. They went after David Price as a free agent, attempted to re-sign Jason Heyward and made a run at Giancarlo Stanton last offseason. Instead, they've settled for secondary pieces, signing Dexter Fowler in 2017 and acquiring Marcell Ozuna last winter.

From the free agents set to cash in to the big-name stars sure to come up in trade rumors all winter long, keep up with all of the latest action.

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Neither of those outfielders produced up to expectations in 2018, however, with Fowler hitting .180/.278/.298, and Ozuna playing through a shoulder issue and hitting .280/.325/.433 with 23 home runs, a big drop from his 2017 numbers with the Marlins. As a result, the Cardinals' offense was good in 2018, but not good enough -- they ranked fifth in the National League in runs, eighth in on-base percentage and seventh in slugging percentage. They needed another big bat.

Enter Goldschmidt. Entering his final season before free agency, he'll be playing his age-31 season in 2019 but remains an excellent all-around player. He's not quite the hitter he was a few years ago -- his OPS peaked at 1.005 in 2015 and was down to .922 in 2018 -- and he's probably lost a step on the bases (32 steals in 2016, but just seven last season), but he still hit .290/.389/.533 with 33 home runs and was worth 5.4 WAR.

Goldschmidt struggled early in 2018 when he was striking out at a prodigious rate. His average dropped to .198 on May 22, but he made some tweaks to his setup and hit .328/.418/.606 the rest of the season. In fact, ESPN Stats & Info reports that from June 1, just Mike Trout, Christian Yelich and J.D. Martinez had a higher OPS than Goldschmidt. Through May 22 he struck out 31.5 percent of the time with a swing-and-miss rate of 31 percent. From May 23 on, he fanned 22.4 percent of the time with a miss rate of 24.5 percent. If the Cardinals get that player over a full season, they just acquired an MVP candidate.

It's a little right-handed, with only Carpenter and Wong from the left side (though the Cardinals could always fix that by entering the Bryce Harper sweepstakes). Carpenter will move to third base on a full-time basis after starting 74 games there in 2018. He actually rated pretty well there at plus-6 defensive runs saved (he's minus-1 at third in his career), so I don't think the Cardinals will take much of a hit on defense and whatever they do lose they should pick up with Goldschmidt's plus glove at first.

Diamondbacks beat writer Nick Piecoro tweeted that Arizona did reach out in an attempt to sign Goldschmidt to an extension. The Cardinals will certainly try a similar tactic and Cardinals fans will be reminded of the Matt Holliday trade in 2009, when the Cardinals acquired him during the season and then re-signed him as a free agent after the season.

Cardinals grade: A-. Look, the Diamondbacks may end up with more long-term WAR with the package of young players, but there's little to suggest the Cardinals will regret the deal as it doesn't seem they gave up any star power. If they can manage to eventually snag Goldschmidt to a long-term deal, that's even better. Still, the deal may ultimately get judged on whether the Cardinals make it back to the postseason after missing it the past three seasons -- the first time that's happened since 1997-99.

Paul Goldschmidt should give the Cardinals the power in 2019 that their lineup has lacked. Jennifer Stewart/Getty Images

Why the Diamondbacks did it: After reaching the division series in 2017, the Diamondbacks were in the division race in 2018 until falling apart the final month and finishing 82-80. The day after losing ace Patrick Corbin in free agency and with center fielder A.J. Pollock also a free agent, they decide to trade their franchise icon and officially retool for 2019. It's not easy trade to make: Goldschmidt is the best position player in franchise history, and a two-time MVP runner-up.

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Did they get enough? Weaver entered 2018 as a breakout candidate after a strong finish in 2017, but failed to keep that momentum and went 7-11 with a 4.95 ERA and 121 strikeouts in 136⅓ innings. The command he showed in 2017 wasn't there and his strikeout rate-minus-walk rate fell from 21.8 percent to 11.0 percent. Weaver's changeup was a big weapon in 2017, limiting batters to a .198 average with a 28 percent strikeout rate, but in 2018 it wasn't as effective (.269, 19 percent K rate) and hitters teed off on his curveball (.562 slugging). There's something here, but without a better weapon to get lefties, he projects as a back-of-the-rotation starter.

Kelly also saw his stock drop in 2018. Once viewed as the heir apparent to Molina, Kelly has picked up just 131 PAs over three seasons with the Cardinals. He hit .269/.378/.395 at Triple-A with 48 walks and 48 strikeouts, but the power has yet to develop. A converted third baseman, he's been catching since 2014 and is still just 24, but there are still concerns that he hasn't mastered the finer points of catching like pitch framing. Still, he controls the strike zone and puts the ball in play and maybe develops into a Francisco Cervelli type with minimal power but a solid OBP.

Young is a 24-year-old second baseman/third baseman who hit .289/.379/.479 with 21 home runs between Class A and Double-A. He actually hit better in Double-A in 35 games and played well in the Arizona Fall League. He already has exceeded expectations for a 37th-round draft pick. There's not much projection here beyond a bench guy. The Diamondbacks also get a draft-pick lottery ticket.

As far as replacing Goldschmidt at first base, the Diamondbacks did sign Eduardo Escobar early in the offseason, so they can shift Jake Lamb over to first base.

Diamondbacks grade: B. They get five years of control of Weaver and six for Kelly and maybe Young surprises. It's not a bad return as they went for volume instead of maybe a player with more upside but a riskier profile. I still like Weaver some and sometimes we give up on these young pitchers too quickly. If he can turn into a No. 3 starter instead of a No. 5, consider that a win for Arizona, with anything Kelly does an added bonus.